GS loses power at 40% throttle

[sector9]

History: Currently I'm on day 7 of a 14 day road trip on the '89 GS. The first 1,200 miles of the trip, the GS has run flawlessly through a variety of terrain along HW 101 along the Pacific Coast through WA, OR, and CA. Stock exhaust, 125mains, 40 pilots, 1 washer, 2 turns (I think) has been the perfect setup with no problems. Cleaned the carbs at the same time that I jetted them, maybe 1k mile before I left.

Precipitating factors: Two nights ago, I gassed up with 87 octane (which I've been doing the whole trip) while the gas station was being filled up by a tanker truck. This was a bad idea, as I have heard that this causes all the sludge and gross stuff in the bottom of the station's tank to get thrown up. Anyways, drove the bike 10-15 miles after that with no issues. Parked it for the night, woke up the next morning for a short ride to a cycle shop, again no issues. Get back on the bike after 10-15min, notice immediately that the bike drops off after ~40% throttle.

Symptoms: At any gear and any speed, as I roll on the throttle, the bike accelerates normally RIGHT until I hit about 40-50% throttle, at which point it falls flat on it's face, bogging and stuttering. It feels exactly like I had just let go of the throttle as soon as I hit the mark. Experimented with snapping open the throttle vs. gradually rolling on, and similar results either way, though rolling on the throttle induces stuttering right below the 40-50% mark.

Attempts at fixing the problem:
1. Drained gas, filled with 91 oct from a different gas station along with some fuel system cleaner. 20 miles ride to cycle new fuel through. No change.
2. Removed carbs, took of float bowls, removed mains, sprayed carb cleaner and compressed air into every orifice/ jets I could find. Reinstalled carbs, airbox, tank. Test rode, no change.
3. Inspected plugs: light brown color, no carbon buildup, no oil.
4. Tested plugs against frame while pressing starter, healthy spark on both sides.
5. checked float height w/ tube method. Both levels slightly above the body-bowl gasket surface.
6. Sprayed carb cleaner around carb-head and carb-airbox mating surfaces with no change in idle.
7. Adjusted idle air screws all the way in, same 50% throttle hesitation. Adj. them 4 turns out, same 50% hesitation.
8. Run on Prime to bypass possible petcock problem, no change.
9. Scratched head, had a beer, slept on it, come back to the bike, called friends who know stuff.

Please, please help. I'm stuck in San Fran at my bro's place, and really want to make it home.

[The Buddha]

OK then remove bowls and poke the jets with the right sized cleaners.
Then, check for air leaks. Dunno, not a lot to go on here.
Cool.
Srinath.

[sector9]

Update: I replaced plugs with brand new NGK DR8EA-9's. Found that I can replicate the problem in neutral by pinning the throttle. It will rev to 6.5k and make a bunch of noise and popping, but won't spin any faster. Also, this is accompanied by some light brownish/blackish smoke.

Srinath, I'll try taking the carbs off again, but where should I focus on for air leaks?

[The Buddha]

OK f%$k that, its got a ripped diaphragm.
Want some, send $20 paypal, I'll have it out tommorow.
The tops, remove them, take off the diaphragm's and look at the sun with it.
Cool.
Srinath.

[ohgood]

man you've got a fun ride ahead. hope you can squeeze in some hiway 1 time.

[sector9]

ohgood, HW 1 has been freaking awesome from the HW 101 junction up north all the way down to San Fran. I really don't want to miss out on Big Sur now that I've come this far, let's hope it's the diaphragms. Checking those this morning...

[sector9]

srinath, the diaphragms look great. when held against the sun, no light shines through. No tears, cracking, holes. Also, the little o-ring under the vacuum nipple on the diaphragm was in place on both carbs.

I've tried running the bike on Prime and "On" with no change in symptoms.

Any other ideas for what it could be?

[coll0412]

Valve clearances?

The black/brown smoke is from it running really rich. Make sure the slides are coming up when you get it to that condition. You can do that by having the airbox off the bike and pinning the throttle and see if the slides are coming up.

I am not really sure why it is super rich, unless there was water in the gas, but if you drained it all out you should be good.

[The Buddha]

Oh, air filter ?? - clean ??
Cool.
Srinath.

[yamahonkawazuki]

what about the "crap " filter ?

[sector9]

Thanks for all the replies everyone. I figured out the problem, though the simplicity of the problem is embarrassing. I've been using a strap mounted tank bag this whole time, with the strap mounted around the black bracket on the fuel tank. This strap was getting sucked over the filter intake on the stock airbox and choking it off. Rerouted the strap under the frame away from the intake, and it's been running like a champ since! Too bad I messed up my perfect idle air screw settings in the process...

Since I got the bike running on Saturday, I rode the Santa Cruz Mountains (HW 9, 35, stopped at Alice's), HW1  through Big Sur, found some great little backroads (G14, HW 25, Route 229, many others). Now I head north bound for the wet, rainy state of Washington. I'm going to miss California...

[beRto]

 

It's always so simple after you figure it out. Congrats on solving the problem!

And good luck in Washington. 

[coll0412]

Thats a good one!